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Here's a video of Experiments 21 and 22 from the Make: Electronics book.

I didn't explain the second experiment (switch de-bouncer) very well, but imagine you have a circuit that counts how many times a button is pressed. You need to de-bounce the switch so that your circuit will only increment the count by one for each press. Without de-bouncing, you will get false increments for each "real" press. Any physical switch (push-button or otherwise) is prone to creating this kind of error, since it is likely to make-and-break several times at the threshold of switching. In fact, by the time you get to this experiment, you will have experienced that phenomenon in previous experiments.